Tsumugi Tōdō is a third-year student at Yamabiko South Academy and the oldest sister to Koyori and Kasane. Her gentle, mature, and grounded nature makes her a respected mediator among peers and a reliable confidante. She embodies the role of eldest sister both within her family and the wider community, always offering courteous guidance and support.

Deeply connected to her rural hometown of Yamabiko, Tsumugi embraces country life with extensive knowledge of its traditions, legends, and history, which she readily shares. This attachment led her to fulfill a personal dream by working part-time at the Yorozuyo Inn, a role demonstrating her appreciation for the community's values.

Physically, she has yellow/blonde hair, brown eyes, and stands 165 cm tall. Her figure, including an F-cup bust size noted as the largest among the female cast, contributes to her status as a "Madonna" figure at school, particularly among male students; she remains unaffected by this attention.

Her relationship with Kazuma Hoshino begins when he inadvertently walks in on her cleaning the bath at Yorozuyo Inn. She reacts calmly to this awkward encounter, which sparks their interactions. Their developing romantic storyline highlights her patience and warmth as she supports Kazuma's adjustment to rural life.

Beyond family and romance, Tsumugi maintains friendships with characters like Hina Sakai and others, reinforcing her role as a unifying presence. Her participation in local events, such as the annual Futogoyama ceremony, underscores her integration into the town's cultural fabric and dedication to its traditions.